In AD&D 1st edition, Oriental Adventures, some weapons are flagged as "entangling", with a footnote marker "g" and the vague footnote, "These weapons make entangling attacks" (page 42). Some of the weapon descriptions, like the chain, chijiriki, kyoketsu-shogi, kawanaga, kusari-gama, and whip, state they can entangle, but I can't find a rules reference for how entangling weapons work. Can anyone point me to these rules?
There are no rules for entangling weapons in Oriental Adventures 1st Edition
Unfortunately, like in some other places in 1e, this is left up to the DM and players to work out. Back then, there were no formalized conditions like we have in more modern editions, and neither Oriental Adventures, nor the 1e PHB or DMG (or even MM) provide any explanation what it means to be entangled by a weapon.
The table refers to the weapon descriptions, but the weapon desciptions themselves, for example for the chain or the kawanaga, again only state these can be used to entangle an opponent, without any exposition of what that means in terms of games mechanics.
So this will be up to you to rule on. Unfortunately, in 1e, even the grappling rules are rather convoluted, and provide little help. The closest we get may be the spell effects of the entangle spell, but even that is of limited use. It states
The grosses, weeds, bushes, and even trees wrap, twist, and entwine about creatures, thus holding them fast for the duration of the spell.
There is no furhter explanation on what "holding them fast" means in terms of game effects either. So you need to do what was common in old school gaming, in an era before the internet: you as the DM make up an interpretation that you think makes sense.
However, the whip also has another use. It can entangle an opponent’s limb(s) or weapon. Any hit scored on an opponent means that an entanglement of some sort will occur. If the whip is in nonproficient hands, the chance is only 5%. In proficient hands, there is a 5% chance per level of the wielder for entanglement, but only at the wielder’s option. The entanglement will be one limb (50%), two limbs (10%), the weapon arm and weapon (20%), or the head (20%, or 40% in the case of an opponent not using a weapon). One-limb entanglement indicates no effective attack for 10 segments. Two-limb entanglement indicates the same, with a 25% chance that the opponent will be kneeling or prone for that period and must thereafter recover its feet. A hit on the weapon arm weapon indicates no effective attack for 10 segments and a 10% chance that the weapon will be lost. (If the weapon is edged, there is a 50% chance that the whip will be severed and useless instead.) A neck hit indicates the opponent can make no effective attack for 10 segments, and additional garrot-type damage will be scored if the opponent has no throat protection such as thick hide, heavy leather, armor, manelike fur or hair, etc. A proficient wielder can disentangle the whip by rolling his or her dexterity or less on 3d6 minus 2. Otherwise, the whip remains entangled until it is broken or severed, or until the wielder pulls it loose, or until the opponent does so (the chance of this last occurrence is 5% + 1% per point of strength of the victim).
Yeah, rather convoluted, and nothing you can remember without having to reference the rules every time.